The present invention relates to improvements in pumps for handling material-carrying liquid, and generally referred to as slurry pumps. More particularly, the invention relates to avoiding premature failure of the seal means for that portion of a slurry pump impeller shaft where it enters the pump shroud or housing.
Slurry pump mechanical shaft seals, for which the slurry liquid is depended upon as lubricant and coolant, are vulnerable to damage from particulate slurry solids which are often quite abrasive. Various expedients have been proposed for alleviating this problem.
One of the more successful schemes for protecting the mechanical seals of slurry pump shafts, especially of the double seal design, has been to flush the area, in which the seal faces operate, by means of pressurized clean flushing liquid, sometimes referred to as barrier fluid, maintained at a pressure level higher than that developed by the pump so that any leakage of the clean liquid is into the slurry. This protects the seal faces from the abrasive action of the slurry solids. However, this scheme has proved to be expensive and requires installation of a pressurizing and flushing liquid circulating system.
With the development of harder slurry materials, such as silicon carbide, attempts have been made to use single seals, without external flushing, in slurry pumps and to attain seal lubrication and cooling by receiving some of the pumped slurry in respective chambers about the seals, through chamber openings exposed to the pumped slurry passing the openings. This has been unsatisfactory for a number of reasons. Generally, the seal chambers have been in spaces which were originally designed for conventional packing, and this has proved to be inadequate because the spaces tended to fill up with destructive packed solids. Another deficiency was that air or vapor tended to collect in the seal areas with deleterious effect on the seals for lack of cooling.
Attempts by the seal manufacturers to alleviate these problems consisted in mounting the seals in flexible arrangements to tolerate shaft excursions which generally occur due to wear deterioration after a time of service of a slurry pump. It was also determined that the space surrounding the seal must be much larger to accommodate pump forced slurry liquid to effect cooling circulation of the seals. These developments improved the reliability of the single slurry pump seals. However, the centrifugal separation effect of the impeller generated vortex in the impeller casing gap from which the lubricating/cooling pumped slurry is received in the seal chamber, still tended to concentrate solid material in the seal area, and caused an insulative air or gas pocket to form around the seal faces, thus restricting the flow of cooling and flushing liquid in the seal chambers. An attempted remedy was the use of so-called "vortex breakers" consisting of ribs protruding into the seal area.
A mechanical slurry pump seal of the kind under discussion depends on having liquid around the seal faces such that not only is there a solids-flushing action but also a sustained liquid film between the faces. The liquid around the faces must remove heat generated by the action of the seal faces. It is therefore important to insure that liquid in motion is always present around the seal faces. But, as has just been explained, this result has not been satisfactorily attained heretofore.